Method and apparatus for sub-pellicle defect reduction on photomasks

ABSTRACT

In one embodiment, the invention is a method and apparatus for sub-pellicle defect reduction on photomasks. One embodiment of a photomask for use in photolithography includes a substrate on which a pattern is formed, the substrate having a frontside and an opposite backside, and a protective coating formed on at least one of the frontside and the backside, the protective coating comprising silicon-based compound.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/685,491, filed Jan. 11, 2010, which in turn claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/143,658,filed Jan. 9, 2009. Both of these applications are herein incorporatedby reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor fabrication,and relates more specifically to photolithographic techniques forfabricating semiconductors.

The present invention relates generally to semiconductor fabrication,and relates more specifically to photolithographic techniques forfabricating semiconductors.

Defect growth during exposure is a growing concern in the photomaskindustry. Surface contaminants from the mask manufacturing process,outgassing from pellicle, adhesives, and storage containers, as well asgeneral airborne contaminations can react during lithographic exposureand form defects on both the frontside (sub-pellicle) and backside ofthe mask. Sub-pellicle defect growth is a growing concern because thedefects are in the focal plane of the mask and can actually print ontothe semiconductor wafer. Although backside defects are not in the focalplane, they are also a concern because a large number of defects cancause a “haze” that affects the transmission of light through the mask.In addition, residual metals and other impurities on the frontside andbackside of the mask may act as nucleation sites for defect growth,because these defects are often composed of heteroatoms that are veryreactive with metals. It is often found that defects are foiined fromcontaminants containing heteroatoms that are very reactive with metalsand other inorganic impurities.

Conventional methods for reducing mask surface contamination include theapplication of fluorocarbon films that require specific tooling, or theapplication of aqueous solutions that prevent large crystal formationbut do not prevent backside haze.

Thus, there is a need in the art for a method and apparatus forsub-pellicle defect reduction on photomasks.

SUMMARY OF THE INVENTION

In one embodiment, the invention is a method and apparatus forsub-pellicle defect reduction on photomasks. One embodiment of aphotomask for use in photolithography includes a substrate on which apattern is formed, the substrate having a frontside and an oppositebackside, and a protective coating formed on at least one of thefrontside and the backside, the protective coating comprisingsilicon-based compound.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention may be had by reference to embodiments, some of which areillustrated in the appended drawings. It is to be noted, however, thatthe appended drawings illustrate only typical embodiments of thisinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1 is a cross sectional view illustrating one embodiment of aphotomask according to the present invention; and

FIG. 2 is a flow diagram illustrating one embodiment of a method forforming a photomask, according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

In one embodiment, the present invention is a method and apparatus forsub-pellicle defect reduction on photomasks. Embodiments of theinvention coat a photomask with a thin film of a silicon-based compound,such as siloxane, silane, silicon oxide, silicon dioxide, siliconnitride, or silicon carbon oxyhydride, which reduces the formation offrontside sub-pellicle defects and backside mask defects on thephotomask.

FIG. 1 is a cross sectional view illustrating one embodiment of aphotomask 100 according to the present invention. As illustrated, thephotomask 100 comprises a plate or substrate 102 on which a pattern (notillustrated) is formed. A pellicle 106 is coupled to the substrate 102for use in photolithographic processing. The pellicle 106 comprises amembrane 108 that is coupled to the substrate 102 by a frame 110, suchthat the membrane 108 is held a fixed distance from the surface of thesubstrate 102.

According to embodiments of the present invention, at least thefrontside of the substrate 102 is coated with a first protective layer104. In a further embodiment, the opposite backside of the substrate 102is coated with a second protective layer 112. The first and secondprotective layers 104 and 112 are designed to minimize defect growth onthe substrate 102, and are thin relative to the substrate 102 (e.g.,approximately ten nanometers or less). In one embodiment, at least oneof the first and second protective layers 104 and 112 comprises anoptically neutral silicon-based compound, such as siloxane, silane,silicon oxide, silicon dioxide, silicon nitride, or silicon carbonoxyhydride.

The first and second protective layers 104 and 112 significantly reducethe occurrence of defects on the substrate 102 of the photomask 100.When the first and second protective layers 104 and 112 are formed ofsilicon oxide or silicon nitride, the first and second protective layers104 and 112 create a diffusion barrier that effectively isolates theadsorbed contamination on the substrate surface from the correspondingairborne contaminants. The first and second protective layers 104 and112 also cover residual metals, other foreign atoms and surface defectson the substrate 102, so that nucleation sites for airbornecontamination are significantly reduced. By contrast, when the first andsecond protective layers 104 and 112 are formed of siloxane or silane,the first and second protective layers 104 and 112 form hydrophobicsurfaces that substantially prevent the adsorption of water vapor, whichhas been implicated as a major factor in defect formation. This limitsthe surface reaction for defect growth. Thus, application of the firstand second protective layers 104 and 112 effectively traps orimmobilizes contaminants on the surface of the substrate 102, preventingthe contaminants from turning into defects.

Experimental results have shown that thicknesses of ten nanometers orless for the first and second protective layers 104 and 112 are thinenough to create a substantially pristine surface on the substrate 102without significantly affecting the optical characteristics of thephotomask 100.

FIG. 2 is a flow diagram illustrating one embodiment of a method 200 forforming a photomask, according to one embodiment of the presentinvention. The method 200 may be implemented, for example, to form thephotomask 100 illustrated in FIG. 1.

The method 200 is initialized at step 202 and proceeds to step 204,where the method 200 patterns a photomask. That is, the method 200 formsa mask pattern on the substrate of the photomask. The method 200 thenproceeds to step 206 and applies an optically neutral silicon-based filmto the photomask (e.g., such as the first and second protective layers104 and 112 described with reference to FIG. 1). The optically neutralsilicon-based film is applied to the frontside of the photomask, thebackside of the photomask, or both. As described above, in oneembodiment, the optically neutral silicon-based film is formed of atleast one of: siloxane, silane, silicon oxide, silicon dioxide, siliconnitride, or silicon carbon oxyhydride. In one embodiment, thecomposition of the optically neutral silicon-based film dictates theapplication method used to coat the photomask.

In one embodiment, where the optically neutral silicon-based film isformed of silicon oxide or silicon dioxide, both the frontside and thebackside of the photomask are coated using either sputter deposition orchemical vapor deposition (CVD). In a further embodiment, the siliconoxide or silicon dioxide coating is applied to a thickness ofapproximately ten nanometers or less. In an alternative embodiment,where the optically neutral silicon-based film is formed of a siloxanecompound, common photomask manufacturing environment equipment, such asa Hexamethyldisilazane oven, is used to spray or dip the photomask in adilute siloxane compound solution. In a further embodiment, the coatingis cured after spraying or dipping of the photomask. In yet anotherembodiment, where the optically neutral silicon-based film is formed ofsilane, the optically neutral silicon-based film is a self-assembledlayer of silane that may be applied, for example, using a gas phase orsolution technique.

The method 200 mounts a pellicle to the photomask in step 208, beforeterminating in step 210.

A protective silicon-based film can thus be applied to one or both sidesof a photomask using equipment that is common to photomask manufacturingenvironments (i.e., substantially no specialized equipment isnecessary). As discussed above, a film applied in this manner creates asubstantially pristine surface for reducing defects on the surface of aphotomask without significantly affecting the optical characteristics ofthe photomask.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof. Various embodiments presentedherein, or portions thereof, may be combined to create furtherembodiments. Furthermore, terms such as top, side, bottom, front, back,and the like are relative or positional terms and are used with respectto the exemplary embodiments illustrated in the figures, and as suchthese terms may be interchangeable.

1. A photomask for use in photolithography, comprising: a substrate onwhich a pattern is formed, the substrate having a frontside and anopposite backside; and a protective coating formed on at least one ofthe frontside and the backside, the protective coating comprising amaterial that isolates adsorbed contaminants residing on the at leastone of the frontside and the backside.
 2. The photomask of claim 1,wherein the protective coating comprises a diffusion barrier.
 3. Thephotomask of claim 2, wherein the protective coating comprises siliconoxide.
 4. The photomask of claim 2, wherein the protective coatingcomprises silicon nitride.
 5. The photomask of claim 1, wherein theprotective coating comprises a hydrophobic surface.
 6. The photomask ofclaim 5, wherein the protective coating comprises siloxane.
 7. Thephotomask of claim 5, wherein the protective coating comprises silane.8. The photomask of claim 1, wherein the protective coating is opticallyneutral.
 9. The photomask of claim 1, wherein the protective coating hasa thickness of approximately ten nanometers or less.
 10. A method forforming a photomask for use in photolithography, the method comprising:patterning a substrate, the substrate having a frontside and an oppositebackside; and applying a protective coating to at least one of thefrontside and the backside, the protective coating comprising a materialthat isolates adsorbed contaminants residing on the at least one of thefrontside and the backside.
 11. The method of claim 10, wherein theprotective coating comprises a diffusion barrier.
 12. The method ofclaim 11, wherein the protective coating comprises silicon oxide. 13.The method of claim 11, wherein the protective coating comprises siliconnitride.
 14. The method of claim 10, wherein the protective coatingcomprises a hydrophobic surface.
 15. The method of claim 14, wherein theprotective coating comprises siloxane.
 16. The method of claim 14,wherein the protective coating comprises silane.
 17. The method of claim10, wherein the protective coating is optically neutral.
 18. The methodof claim 10, wherein the protective coating has a thickness ofapproximately ten nanometers or less.
 19. A photomask for use inphotolithography, comprising: a substrate on which a pattern is formed,the substrate having a frontside and an opposite backside; and aprotective coating formed on at least one of the frontside and thebackside, the protective coating comprising an optically neutralsilicon-based film.
 20. The photomask of claim 19, wherein theprotective coating has a thickness of approximately ten nanometers orless.